Cluster ion exchange membrane and electrolyte membrane electrode connection body

ABSTRACT

The present invention provides a composite ion exchange membrane which has high mechanical strength and is suitable for use as a solid polymer electrolyte membrane excellent in ionic conductivity and a method for its production. The invention is achieved by a composite ion exchange membrane including a composite layer comprising a support membrane with continuous voids formed of polybenzazole polymer, the support membrane being impregnated with ion exchange resin, and surface layers formed of ion exchange resin free of support membranes, the surface layers being formed on both surfaces of the composite layer so as to sandwich the composite layer therebetween.

TECHNICAL FIELD

The present invention relates to a composite ion exchange membranesuperior in mechanical strength and ionic conductivity and,particularly, to a solid polymer electrolyte membrane and an electrolytemembrane-electrode assembly.

BACKGROUND ART

In late years much attention has been focused on novel power generatingtechniques which are superior in energy efficiency or environmentalfriendliness. In particular, solid polymer fuel cells using solidpolymer electrolyte membranes are characterized as exhibiting highenergy density and being started and stopped more easily than fuel cellsof other systems due to their lower operating temperature. Therefore,they are on development as generators for electric motorcars, dispersedpower generation and the like. In addition, development of directmethanol fuel cells which use solid polymer electrolyte membranes andinto which methanol is supplied directly as fuel are underway forapplications such as electric sources of portable devices.Proton-conducting ion exchange resin films are usually used for solidpolymer electrolyte membranes. Solid polymer electrolyte membranes arerequired to have characteristics such as fuel permeation inhibitabilityand mechanical strength preventing permeation such as hydrogen of fuelis necessary for a solid polymer electrolyte membrane as well as protonconductivity. As such a solid polymer electrolyte membrane, for example,perfluorocarbon sulfonic acid polymer membranes in which sulfonic acidgroups are introduced, typified by Nafion (commercial name) manufacturedby Du Pont, U.S.A., are known.

In order to enhance output power and efficiency of solid polymer fuelcells, it is effective to reduce the ion conduction resistance of solidpolymer electrolyte membranes. One of the measures for it is to reducethe thickness of membranes. For solid polymer electrolyte membranestypified by Nafion, reduction of membrane thickness has been attempted.However, those membranes have problems in that when the thickness isreduced, the mechanical strength decreases and, as a result, when asolid polymer electrolyte membrane and an electrode are bonded togetherby hot pressing, the membrane easily ruptures or, due to the change ofdimensions of the membrane, the electrode bonded to the solid polymerelectrolyte membrane peels off, resulting in lowering of electric powergeneration characteristics. Moreover, they have problems in that thereduction of the thickness results in lowering of the fuel permeationinhibitability, which reduces electromotive force or efficiency for fuelutilization.

Moreover, solid polymer electrolyte membranes can be applied for a widevariety of applications including, in addition to an application as anion exchange resin membrane of fuel cells shown above, applications inthe field of electrochemistry such as electrolytic applications e.g.alkali electrolysis and production of hydrogen from water andelectrolyte applications in various types of cells e.g. lithium cellsand nickel hydrogen cells; mechanical functional material applicationssuch as microactuators and artificial muscles, applications forfunctional materials for recognizing/responding to ions, molecules andthe like; and applications for functional materials forseparation/purification. It is conceivable that, in each application,new, superior functions will be offered by increasing the strength ofsolid polymer electrolyte membranes or reducing the thickness thereof.As a method for improving the mechanical strength of a solid polymerelectrolyte membrane and controlling its dimensional change, compositesolid polymer electrolyte membranes resulting from combining solidpolymer electrolyte membranes with various types of reinforcingmaterials have been proposed. Patent Document 1 discloses a compositesolid polymer electrolyte membrane prepared by allowing aperfluorocarbon sulfonic acid polymer, which is an ion exchange resin,to soak into voids of a drawn porous polytetrafluoroethylene membraneand uniting them. However, these composite solid polymer electrolytemembranes have problems in that the reinforcing material is easilysoftened by the heat generated during electric power generation becauseit is made of polytetrafluoroethylene and, therefore, the membrane tendsto change in dimension due to creep, that when the reinforcing materialis impregnated with perfluorocarbon sulfonic acid polymer solution andthen dried, the capacity of the voids in the reinforcing material causesalmost no change and, therefore, the perfluorocarbon sulfonic acidpolymer solidified in the voids of the reinforcing material tends to beunevenly distributed, that to fill the voids completely with the polymerrequires a complicated process such as repeating twice or more theprocess of impregnation with the ion exchange resin solution and drying,and that it is difficult to obtain a membrane with a superior fuelpermeation inhibitability because voids tend to remain. Patent Document2 discloses a composite solid polymer electrolyte membrane in whichfibrillized polytetrafluoroethylene as a reinforcing material isdispersed in a membrane made of perfluorocarbon sulfonic acid polymer.However, the composite solid polymer electrolyte membrane has a problemin that delamination of the electrodes occurs because the membrane cannot exhibit sufficient mechanical strength and therefore the deformationof the membrane can not be controlled because the membrane has astructure where the reinforcing material is discontinuous. Moreover,Patent Document 3 discloses an electrolyte whose creep elongation athigh temperatures is reduced through improvement in heat resistanceachieved by crosslinking side chains multifunctionalized. However, thecreep elongation of the electrolyte in Patent Document 3 is definedbased on a deformation occurring during a very short time as short asfour minutes and there is no discussion about the effect of moisture.When it is exposed to a high temperature, humidified atmosphere for along period of time, lowering of heat resistance caused by decompositionof the side chain crosslinking structure introduced or deformationcaused by relaxation of a secondary structure of a main chain areunavoidable. Therefore, a measure such as that described in PatentDocument 3 has a problem in that no electrolyte can be achieved whichexhibits a small creep deformation under a load applied for a longperiod of time under a high temperature, humidified atmosphere importantfor practical use of solid polymer fuel cells.

Polybenzazole polymers such as polybenzooxazole (PBO) andpolybenzimidazole (PBI) are expected to be suitable as a reinforcingmaterial of solid polymer electrolyte membranes because they aresuperior as having a high heat resistance, a high strength and a highelastic modulus Patent Document 4 discloses solid polymer electrolytemembranes in which a PBO porous membrane is combined with various typesof ion exchange resin. However, it has problems in that on both surfacesof a PBO porous membrane obtained by a method including solidifying,directly in a water bath, a PBO solution film formed from a dope whichexhibits mesomorphism such as that disclosed in that document, denselayers having less apertures are formed; when the membrane is combinedwith ion exchange resin, an ion exchange resin solution is difficult tobe soaked into the membrane, resulting in a low content of the ionexchange resin in a composite membrane, and characteristics such asionic conductivity inherent to the ion exchange resin are greatlydeteriorated. Moreover, the composite ion exchange membrane disclosed inthis document is not particularly restricted with respect to theformation or thickness of surface ion exchange resin layers. However,the presence and thickness of surface layers in a composite ion exchangemembrane have an effect on adhesion between an ion exchange resinserving as a binder and an ion exchange resin forming solid polymerelectrolyte membranes and it is important to optimally control them.

Patent Document 5 discloses a method for manufacturing a polymer filmfor fuel cells, in the film an acid being trapped in voids of a PBIporous membrane. However, a film trapping a free acid therein obtainedby a method such as that described in this document has problems in thatits ionic conductivity in a low temperature range such as that up to100° C. is lower than that of ion exchange resin membranes such as theaforementioned Nafion and that the acid tends to exude. Moreover, PatentDocument 6 discloses a method for obtaining a polybenzazole film byforming a film from an optically anisotropic polybenzazole polymersolution and solidifying the film through a process of renderingisotropic. However, a polybenzazole film obtained by a method such asthat disclosed in this document is a transparent and highly dense film,which is not suitable for the purpose of converting it to an ionexchange membrane by impregnating it with ion exchange resin.

[Patent Document 1]

Japanese Patent Laying-Open No. 8-162132

[Patent Document 2]

Japanese Patent Laying-Open No. 2001-35508

[Patent Document 3]

Japanese Patent Laying-Open No. 2002-324559

[Patent Document 4]

Pamphlet of International Publication No. WO00/22684

[Patent Document 5]

Pamphlet of International Publication No. WO98/14505

[Patent Document 6]

Japanese Patent Laying-Open No. 2000-273214

The present invention provides a composite ion exchange membrane whichhas high mechanical strength and is suitable for use as a solid polymerelectrolyte membrane excellent in ionic conductivity and a method forits production and, furthermore, an electrolyte membrane-electrodeassembly having good adhesion between the electrolyte membrane and theelectrode assembly.

DISCLOSURE OF THE INVENTION

The present invention provides a composite ion exchange membraneincluding a composite layer comprising a support membrane withcontinuous voids formed of polybenzazole polymer, the support layerbeing impregnated with ion exchange resin, and surface layers formed ofion exchange resin free of support membranes, the surface layers beingformed on both surfaces of the composite layer so as to sandwich thecomposite layer therebetween. Moreover, the present invention alsoprovides a composite ion exchange membrane including a composite layercomprising a support membrane with continuous voids formed ofpolybenzazole polymer, the support layer being impregnated with ionexchange resin, and surface layers formed of ion exchange resin free ofsupport membranes, the surface layers being formed on both surfaces ofthe composite layer so as to sandwich the composite layer therebetween,wherein at least one surface of the support membrane has an open arearatio of 40% or more. Moreover, the present invention provides acomposite ion exchange membrane including a composite layer comprising asupport membrane with continuous voids formed of polybenzazole polymer,the support layer being impregnated with ion exchange resin, and surfacelayers formed of ion exchange resin free of support membranes, thesurface layers being formed on both surfaces of the composite layer soas to sandwich the composite layer therebetween, wherein the compositeion exchange membrane exhibits an creep elongation of up to 50% when itis applied with a load of 1 MPa for two hours under a dry atmosphere at130° C. Moreover, the present invention provides a composite ionexchange membrane including a composite layer comprising a supportmembrane with continuous voids formed of polybenzazole polymer, thesupport layer being impregnated with ion exchange resin, and surfacelayers formed of ion exchange resin free of support membranes, thesurface layers being formed on both surfaces of the composite layer soas to sandwich the composite layer therebetween, wherein the compositeion exchange membrane exhibits a creep elongation in high temperaturewater of up to 60% when it is applied with a load of 1 MPa for two hoursin water at 130° C. Moreover, the present invention provides a methodfor producing a support membrane, the method comprising forming apolybenzazole polymer solution into a film form and then solidifying it,wherein the polybenzazole polymer solution is an isotropic solutioncontaining the polybenzazole polymer in an amount of from 0.5% by weightto 2% by weight. Furthermore, the present invention provides anelectrolyte membrane-electrode assembly comprising a composite ionexchange membrane including a composite layer comprising a supportmembrane with continuous voids formed of polybenzazole polymer, thesupport layer being impregnated with ion exchange resin, and surfacelayers formed of ion exchange resin free of support membranes, thesurface layers being formed on both surfaces of the composite layer soas to sandwich the composite layer therebetween, wherein catalyst layersand gas diffusion layers are disposed on both surfaces of the compositeion exchange membrane.

The polybenzazole polymer used as the support membrane of the presentinvention refers to polymers having a structure containing an oxazolering, a thiazole ring and an imidazole ring in the polymer chain andspecifically to polymers containing a repeating unit represented by thefollowing general formulas in the polymer chain.

Here, Ar₁, Ar₂ and Ar₃ each represent an aromatic unit, which may have asubstituent such as various types of aliphatic group, aromatic group,halogen group, hydroxyl group, nitro group, cyano group andtrifluoromethyl group. These aromatic units may be monocyclic units suchas benzene ring, condensed ring units such as naphthalene, anthraceneand pyrene, and polycyclic aromatic units in which such aromatic unitsare linked via two or more arbitrary bonds. The positions of N and X inaromatic units are not particularly restricted if a configuration suchthat a benzazole ring can be formed is established. Moreover, these maybe heterocyclic aromatic units containing N, O, S or the like inaromatic rings as well as hydrocarbon aromatic units. X represents O, Sand NH.

The aforementioned Ar₁ is preferably ones represented by the followinggeneral formulas.

Here, Y₁ and Y₂ each represent CH or N, and Z represents a direct bond,—O—, —S—, —SO₂—, —C(CH₃)₂—, —C(CF₃)₂— and —CO—.

The aforementioned Ar2 is preferably one which is represented by thefollowing general formulas.

Here, W represents —O—, —S—, —SO₂—, —C(CH₃)₂—, —C(CH₃)₂— and —CO—.

The aforementioned Ar₃ is preferably one which is represented by thefollowing general formula.

These polybenzazole polymers may be homopolymers having the foregoingrepeating units. Alternatively, they also may be random, alternating orblock copolymers comprising a combination of the abovementionedstructural units. examples of which include those disclosed in U.S. Pat.Nos. 4,703,103, 4,533,692, 4,533,724, 4,533,693, 4,539,567 and4,578,432.

Specific examples of such polybenzazole structural units include onesrepresented by the following structural formulas.

In addition, not only these polybenzazole structural units, but alsorandom, alternating or block copolymers with additional polymerstructural units are available. In such a situation, the additionalpolymer structural units are preferably chosen from aromatic polymerstructural units with superior heat resistance. Specific examplesinclude polyimide structural units, polyamide structural units,polyamideimide structural units, polyoxydiazole structural units,polyazomethine structural units, polybenzazoleimide structural units,polyetherketone structural units and polyethersulfone structural units.

Examples of the polyimide structural units include ones represented bythe following general formula.

Here, Ar₄ is represented by a tetravalent aromatic unit. Preferred arethose represented by the following structures.

Ar₅ is a divalent aromatic unit and preferred are those represented bythe following structures. On the aromatic rings shown here, variouskinds of substituents may be present such as a methyl group, a methoxygroup, a halogen group, a trifluoromethyl group, a hydroxyl group, anitro group and a cyano group.

Specific examples of these polyimide structural units include onesrepresented by the following structural formulas.

Examples of polyamide structural units include ones represented by thefollowing structural formulas.

Here, Ar₆, Ar₇ and Ar₈ are preferably each independently one which ischosen from the following structures. On the aromatic rings shown here,various kinds of substituents may be present such as a methyl group, amethoxy group, a halogen group, a trifluoromethyl group, a hydroxylgroup, a nitro group and a cyano group.

Specific examples of these polyamide structural units include onesrepresented by the following structural formulas.

Examples of polyamideimide structural units include ones represented bythe following structural formulas.

Here, Ar₉ is preferably chosen from the structures provided above asspecific examples of Ar₅.

Specific examples of these polyamideimide structural units include onesrepresented by the following structural formulas.

Examples of polyoxydiazole structural units include ones represented bythe following structural formulas.

Here, Ar₁₀ is preferably chosen from the structures provided above asspecific examples of Ar₅.

Specific examples of such polyoxydiazole structural units include onesrepresented by the following structural formulas.

Examples of polyazomethine structural units include ones represented bythe following structural formulas.

Here, Ar₁₁ and Ar₁₂ are preferably chosen from the structures providedabove as specific examples of Ar₆.

Specific examples of these polyazomethine structural units include onesrepresented by the following structural formulas.

Examples of polybenzazoleimide structural units include ones representedby the following structural formulas.

Here, Ar₁₃ and Ar₁₄ are preferably chosen from the structures providedabove as specific examples of Ar₄.

Specific examples of such polybenzazoleimide structural units includeones represented by the following structural formulas.

Polyetherketone structural units and polyethersulfone structural unitsare structural units generally having a structure in which aromaticunits are combined via a ketone bond or a sulfone bond as well as anether bond, which include structural components selected from thefollowing structural formulas.

Here, Ar₁₅ through Ar₂₃ are preferably each independently onesrepresented by the following structures. On the aromatic rings shownhere, various kinds of substituents may be present such as a methylgroup, a methoxy group, a halogen group, a trifluoromethyl group, ahydroxyl group, a nitro group and a cyano group.

Specific examples of these polyetherketone structural units include onesrepresented by the following structural formulas.

The aromatic polymer structural units which can be copolymerizedtogether with these polybenzazole polymer structural units do not referexactly to repeating units in polymer chains, but refer to structuralunits which can be present in polymer chains together with polybenzazolestructural units. With respect to these copolymerizable aromatic polymerstructural units, not only a single kind of units but also two or morekinds of units may be copolymerized in combination. Such copolymers canbe synthesized by introducing amino groups, carboxyl groups, hydroxylgroups, halogen groups or the like at unit terminals formed ofpolybenzazole polymer structural units, followed by polymerizing theresultant as reaction components in the synthesis of those aromaticpolymers, or introducing carboxyl groups at unit terminals of thosearomatic polymer structural units, followed by polymerizing theresultant as reaction components in the synthesis of polybenzazolepolymer.

The aforementioned polybenzazole polymer is obtained throughcondensation polymerization in polyphosphoric acid solvent. The degreeof polymerization of the polymer, which is expressed using intrinsicviscosity, is from 15 dL/g to 35 dL/g, preferably from 20 dL/g to 26dL/g. That below this range is unfavorable because a resulting supportmembrane have low strength. On the other hand, that over the range isalso unfavorable because the concentration range of a polybenzazolepolymer solution from which an isotropic solution can be formed islimited and it is difficult to form a film under isotropic conditions.

Methods available for forming a film of a polybenzazole polymer solutioninclude, in addition to a film formation method, called the castingmethod, in which a polymer solution is cast on a substrate using adoctor blade or the like, any method in which a solution is formed intoa film form, e.g. a method comprising extruding through a linear slitdie, a method comprising blow extruding through circular slit die, asandwich method comprising pressing a polymer solution sandwichedbetween two substrates through a roller, and spin coating. Preferredfilm formation methods suitable for the purpose of the present inventionare the casting method and the sandwich method. As a substrate plate forthe casting method or a substrate for the sandwich method, glass plates,metal plates, resin films and the like can be used. In addition, for thepurpose of controlling the void structure of a support membrane atsolidification, various types of porous material can be preferablyemployed as a substrate or substrate plate.

In order to obtain a support membrane which is uniform and which has ahigh porosity, it is important to form the polybenzazole polymersolution used in the present invention into a film at a compositionunder isotropic conditions. A preferred concentration range of thepolybenzazole polymer solution is from 0.5% to 2%, more preferably from0.8% to 1.5%. Concentrations lower than this range are unfavorablebecause the polymer solution comes to have a low viscosity and thereforefilm formation methods which can be applied are restricted and alsobecause a resulting support membrane comes to have a small strength. Onthe other hand, concentrations higher than that range are unfavorablebecause no support membranes having high porosities are obtained and thesolution exhibits anisotropy at some polymer compositions or degrees ofpolymerization of the polybenzazole polymer.

In order to adjust the concentration of the polybenzazole polymersolution within the above range, methods shown below can be employed. Amethod comprising separating polymer solid temporarily from apolymerized polybenzazole polymer solution and then adding solvent againto dissolve the solid, thereby adjusting the concentration. A methodcomprising adding solvent to a polymer solution resulting directly fromcondensation polymerization in polyphosphoric acid without separatingpolymer solid therefrom, thereby diluting the solution to adjust theconcentration. A method comprising adjusting the polymerizationcomposition of the polymer to obtain directly a polymer solution havinga concentration within the aforementioned range.

Examples of solvents suitably used for adjusting the concentration ofthe polymer solution include methanesulfonic acid, dimethylsulfuricacid, polyphosphoric acid, sulfuric acid and trifluoroacetic acid. Mixedsolvents comprising combinations of these solvents are also used. Aboveall, methanesulfonic acid and polyphosphoric acid are particularlypreferred.

As a method for realizing the porous structure of the support membrane,a method is used which comprises contacting an isotropic polybenzazolepolymer solution in a film form with a poor solvent to solidify it. Thepoor solvent is a solvent which is miscible with the solvent in thepolymer solution. It may be either in a liquid phase or in a gas phase.In addition, a combination of solidification using a poor solvent in agas phase and solidification using a poor solvent in a liquid phase canalso be employed suitably. As the poor solvent to be used for thesolidification, water, aqueous solutions of acids, aqueous solutions ofinorganic salts, organic solvents such as alcohol, glycol and glycerin,and so on can be used. However, particular caution is required in choiceof the poor solvent used for the solidification because in somecombinations with the polybenzazole polymer solution to be used,problems will arise, for example, the support membrane comes to have asmall surface open area ratio or a small porosity, or discontinuousvoids are formed inside the support membrane. In the solidification ofan isotropic polybenzazole polymer solution in the present invention,the structures and the porosities of the surface and the inside of thesupport membrane are controlled successfully by choosing solidificationconditions and a poor solvent among water vapor, aqueous solution ofmethanesulfonic acid, aqueous solution of phosphoric acid, aqueoussolution of glycerin and aqueous solutions of inorganic salts such asaqueous solution of magnesium chloride. Particularly preferred methodsfor solidification include a method comprising contacting the solutionwith water vapor to solidify it, a method comprising contacting thesolution with water vapor for a short period of time in the early stageof solidification and then contacting it with water, and a methodcomprising contacting the solution with an aqueous solution ofmethanesulfonic acid.

The support membrane tends to shrink with progress of solidification ofthe polymer. During the progress of the solidification, a tenter or afixing frame may be used for preventing the formation of wrinkles causedby uneven shrinkage of the support membrane. Moreover, in the case ofsolidifying a polymer solution shaped on a substrate plate such as aglass plate, the shrinkage on the substrate plate may be inhibited bycontrolling the roughness of the surface of the substrate plate.

It is desirable that the support membrane solidified in the mannermentioned above be fully washed for avoidance of problems such asacceleration of decomposition of the polymer caused by remaining solventor spill of remaining solvent during the use of a composite electrolytemembrane. The washing can be performed through immersion of the supportmembrane in washing liquid. Particularly desirable washing liquid iswater. It is desirable that the washing with water be carried out untilthe washings come to have pH within the range of from 5 to 8, moredesirably from 6.5 to 7.5.

By use of an isotropic polybenzazole polymer solution having with aconcentration within the above-mentioned specific range and use of aproper solidification method selected from the methods mentioned above,a support membrane made of a polybenzazole polymer having a structuresuitable for the purpose of the present invention can be obtained. It isa porous support membrane with continuous voids having openings in atleast one surface of the support membrane. The facts that the supportmembrane has three-dimensional network structure made of fibril fibersof polybenzazole polymer and that it has three-dimensionally continuousvoids were confirmed by observation of the surface of the supportmembrane in water using an atomic force microscope and bycross-sectional observation using transmission electron microscopicobservation of the support membrane holding its structure in water byepoxy embedding-epoxy removal. Japanese Patent Laying-OpenNo.2002-203576 discloses an electrolyte membrane comprising a membranesupport having open pores penetrating in the thickness direction of themembrane and ion-conducting material introduced into the support.However, it is undesirable for a support in which the direction of theopen pores is restricted mainly to the thickness direction of themembrane such as that disclosed in the publication to be used for anelectrolyte membrane of a fuel cell because of problem, for example, inthat during its use as anion exchange membrane of a fuel cell,occurrence of conditions uneven in both directions of the membraneregarding concentration distribution of fuel gas and amount of adheringelectrode catalyst caused by a small continuity of the ion-conductingmaterial in both directions of the membrane tends to cause a localdeterioration of the ion exchange membrane.

The porosity of the support membrane of the present invention ispreferably 90% by volume or more, and more preferably 95% by volume ormore. A porosity under this range is undesirable because combining ofthe membrane with ion exchange resin results in a small content of theion exchange resin, which leads to a reduced ionic conductivity.

The support membrane of the present invention preferably has at leastone surface having a surface open area ratio of 40% or more, morepreferably 50% or more, and particularly preferably 60% or more. A casewhere at least one surface has a surface open area ratio under thisrange is unfavorable because ion exchange resin becomes difficult tosoak into voids of the support membrane when the support membrane andthe ion exchange resin are combined.

Described below is a method to obtain a composite ion exchange membraneby combining ion exchange resin with the porous support membrane made ofa polybenzazole polymer obtained in the method described above. Themethod is one which includes immersing the support membrane in an ionexchange resin solution without drying it to allow the ion exchangeresin solution to displace the liquid inside the support membrane,thereby obtaining a composite ion exchange membrane. In the case wherethe liquid inside the ion exchange membrane has a solvent compositiondifferent from that of the ion exchange resin solution, a method whichincludes allowing the liquid inside to be displaced in advance inconformity to the solvent composition of the ion exchange resin solutioncan also be applied.

The support membrane of the present invention has a characteristic inthat the void structure shrinks and the apparent volume of the supportmembrane decreases greatly with decrease of the liquid inside the voidscaused by drying. In the case where the support membrane is dried undercontrol of its shrinkage in both directions by fixing it, for example,in a metal frame without allowing ion exchange resin to soak in thesupport membrane, a shrinkage occurs in the thickness direction and theapparent thickness of the support membrane after the drying is withinthe range of from 0.5% to 10% of the thickness before the drying. Poroussupport membranes other than the support membrane of the presentinvention, for example, support membranes made of drawnpolytetrafluoroethylene polymer porous membranes do not cause such agreat shrinkage.

When the liquid inside the voids of the support membrane is displaced byion exchange resin solution and then the solution is dried, the supportmembrane shrinks as the volume of the ion exchange resin solutiondecreases through evaporation of the solvent of the ion exchange resinsolution contained in the voids due to the above-mentionedcharacteristics of the support membrane. Therefore, a dense compositemembrane structure where the voids in the support membrane are filledwith the crystallized ion exchange resin can be obtained easily. Due tothis composite membrane structure, the composite ion exchange membraneof the present invention exhibits a superior fuel permeationinhibitability. Porous support membranes other than the support membraneof the present invention, for example, support membranes made of drawnpolytetrafluoroethylene polymer porous membranes are undesirable becauseeven if the solvent of an ion exchange resin solution soaked into voidsevaporates and, as a result, the volume of the ion exchange resinsolution decreases, this will be accompanied by a small shrinkage of asupport membrane and, therefore, many voids which are not filled withion exchange resin will be formed inside the composite membrane afterdrying and, moreover, no surface layers free of supports are formed onboth sides of the support membrane.

In addition, because the support membrane shrinks greatly, adjustment ofthe combination of the concentration and the viscosity of the ionexchange resin solution and physical properties of the solvent such asvolatility with the thickness, the porosity and the like of the supportmembrane will lead to the formation of the composite layer in whichvoids in the support membrane are filled with the ion exchange resin andalso to the formation of ion exchange resin layers free of the supports,which formation is caused when the excess ion exchange resin solutionadhering to both sides of the support membrane and the ion exchangeresin solution excreted from the inside of the support membrane with theshrinkage of the support membrane are allowed to dry outside thesurfaces of the support membrane. As a result, in the composite ionexchange membrane, a structure where the surface layers are formed ofion exchange resin free of support membranes, the surface layers beingformed on both surfaces of the composite layer so as to sandwich thecomposite layer therebetween, can be realized easily.

Porous support membranes other than the support membrane of the presentinvention, for example, porous support membranes ofpolytetrafluoroethylene polymer do not cause great shrinkage asmentioned above. Therefore, even if ion exchange resin forms solidsinside the support membrane in the course of impregnating it with an ionexchange resin solution and drying, voids will remain intact and no ionexchange resin layer sandwiching the support membrane composite layer.The elimination of this situation requires repeating the impregnationwith the ion exchange resin solution and the drying twice or more, whichwill adversely complicate the process.

The ion exchange resin for use in the composite ion exchange membrane ofthe present invention is not particularly restricted. Examples ofapplicable ion exchange resin include, in addition to the aforementionedperfluorocarbon sulfonic acid polymer, at least one ionomer selectedfrom polystyrene sulfonic acid, poly (trifluorostyrene) sulfonic acid,polyvinyl phosphonic acid, polyvinyl carboxylic acid and polyvinylsulfonic acid polymer, and ionomer resulting from sulfonation,phosphonation or carboxylation of at least one substance selected fromaromatic polymers such as polysulfone, polyphenylene oxide,polyphenylene sulfoxide, polyphenylene sulfide, polyphenylene sulfidesulfone, polyparaphenylene, polyphenylquinoxaline, poly arylketone,polyetherketone, polybenzazole and polyaramide polymer. The polysulfonepolymer referred to herein include at least one selected frompolyethersulfone, polyarylsulfone, polyarylethersulfone,polyphenylsulfone and polyphenylenesulfone polymer. The polyetherketonepolymer referred to herein include at least one selected frompolyetheretherketone, polyetherketone-ketone,polyetheretherketone-ketone and polyetherketone ether-ketone polymer.

The solvent in the ion exchange resin solution described above may beselected from solvents which dissolve ion exchange resin withoutdissolving, decomposing or extremely swelling polybenzazole polymersupport membrane. Since the support membrane is impregnated with the ionexchange resin solution and then the ion exchange resin is precipitatedthrough removal of the solvent, the solvent is preferably one which canbe removed, for example, by being evaporated by means of heating orpressure reduction. The fact that a composite ion exchange membrane canbe produced using an ion exchange resin solution containing ahigh-boiling solvent which can not be used in the preparation of acomposite ion exchange membrane using a support membrane made ofpolytetrafluoroethylene which exhibits a creep at temperatures of about100° C. or higher since the polybenzazole polymer support membrane ofthe present invention has high heat resistance is a superior featurefrom the viewpoint that many kinds of ion exchange resin can be chosen.

The concentration of the ion exchange resin solution and the molecularweight of the ion exchange resin described above are not particularlyrestricted They are chosen appropriately according to the kind of theion exchange resin and the thickness of the composite ion exchangemembrane to be obtained.

The content of the ion exchange resin in the composite ion exchangemembrane obtained in the manner mentioned above is desirably 50% byweight or more, and more desirably 80% by weight or more. A contentunder this range is unfavorable because it results in a large conductiveresistance or a reduced water retentivity of a membrane and thereforesufficient generation performance can not be obtained.

The composite ion exchange membrane of the present invention ischaracterized by having surface layers formed of ion exchange resin freeof supports, the surface layers being formed on both surfaces of thecomposite layer so as to sandwich the composite layer therebetween asdescribed above. Since the composite ion exchange membrane has thecomposite layer and the surface layers, the composite ion exchangemembrane has a high mechanical strength and has the advantage that whenan electrode layer is formed on its surface, it exhibits a superioradhesion with the electrode layer. Each of the surface layers preferablyhas a thickness of from 1 μm to 50 μm which does not exceed half thetotal thickness of the composite ion exchange membrane. The situationwhere the surface layers have thicknesses under that range isundesirable because, for example, the adhesion with the electrode layerworsens and the ionic conductivity decreases On the other hand, thesituation where the surface layers have thicknesses over theabove-mentioned range is undesirable because, for example, since thereinforcing effect by the composite layer does not reach the outermostlayer of the composite ion exchange membrane, only the surface layersswell greatly to peel off from the composite layer when the compositeion exchange membrane absorbs moisture. A more desirable range of thethickness of the surface layers is from 2 μm to 30 μm.

For the purpose of further improving characteristics of the compositeion exchange membrane, such as mechanical strength, ionic conductivityand peeling resistance of the ion exchange resin layers formed on thesurfaces, a method in which the composite ion exchange membrane issubjected to heat treatment under appropriate conditions may also bepreferably employed. In addition, in order to adjust the thickness ofthe ion exchange resin surface layers formed on the surfaces, availableare a method in which the amount of the ion exchange resin layer adheredis increased by immersing the composite ion exchange membrane further inan ion exchange resin solution or applying the ion exchange resinsolution to the composite ion exchange membrane, followed by drying, ora method in which the amount of the ion exchange resin layer adhered isdecreased by immersing the composite ion exchange membrane in an ionexchange resin solution, followed by subjecting part of the ion exchangeresin solution which adhered to the surface of the support membrane toscrape with a scraper, an air knife or a roller or to absorb with amaterial capable of absorbing the solution, such as filter paper andsponge. Alternatively, a method to improve the adhesion of the ionexchange resin layer by hot pressing is also used in combination.

The composite ion exchange membrane of the present invention exhibits anelongation when it is applied with a load of 1 MPa for two hours under adry atmosphere at 130° C., namely a high-temperature creep elongation,of up to 50% and preferably up to 30%. A high-temperature creepelongation over the above-mentioned range is unfavorable because whenthe composite ion exchange membrane is used in a solid polymer fuelcell, a great deformation of the membrane is caused by heat or pressureduring electric power generation and this will cause breakage of thecomposite ion exchange membrane or delamination of electrode layersadhered to the ion exchange membrane. Ion exchange membranes whose heatresistance has been improved by crosslinking of electrolyte such asthose of the prior art shown above exhibit resistance to creep caused bya load for a short time of about four minutes. However, they areunfavorable because they exhibit an insufficient effect of inhibiting ahigh-temperature creep elongation to a load of a long period of time.Moreover, the composite ion exchange membrane of the present inventionexhibits an elongation when it is applied with a load of 1 MPa for twohours in water at 130° C., namely a creep elongation in high temperaturewater, of up to 60%, more desirably up to 40%. If the creep elongationin high temperature water is in the range provided above, the damagecaused by deformation of an ion exchange membrane or the delamination ofan electrode layer adhered to the ion exchange membrane can be inhibitedor the lowering of ionic conductivity caused by swelling of the ionexchange membrane can be prevented even if fuel cells are practicallyoperated under a high temperature, humidified atmosphere. A creepelongation in high temperature water over the range provided above isunfavorable because ion exchange membranes deform greatly or the ionicconductivity is lowered during its operation under a high temperature,humidified atmosphere.

The composite ion exchange membrane of the present invention is superiorin mechanical strength while having a high ionic conductivity. Makingthe most of these characteristics, it is possible to use it as acomposite ion exchange membrane, particularly a solid polymerelectrolyte membrane for solid polymer fuel cells.

The electrode to be used in the present invention is not particularlyrestricted and may be appropriately chosen according to the shape andapplication of fuel cells. For example, carbon paper, carbon cloth andthe like may be chosen. In addition, materials water-repelled with PTFEor the like may also be used. The electrode is not particularlyrestricted with respect to its thickness, but preferred are those havinga thickness of up to 400 μm. Those having a thickness of up to 300 μmare more preferred.

The catalyst layer used in the present invention refers to a layerhaving a catalyst such as platinum. The catalyst is not particularlyrestricted if it can be used for solid polymer fuel cells. It ispreferable that a catalyst of metal, such as platinum, or alloy becontained in the catalyst layer in a state where it is carried on carbonblack. The catalyst layer contains ion exchange resin. The ion exchangeresin may be the same as or different from, but is preferably the sameas, that contained in the composite ion exchange membrane. Two or morekinds of ion exchange resin may be contained. The catalyst layer maycontain, in addition to ion exchange resin, other resins such asfluororesin. Moreover, it may contain free carbon black carrying nocatalyst.

The method for fabricating the electrolyte membrane-electrode assemblyof the present invention is not particularly restricted and may beappropriately chosen according to the shape and application of fuelcells. Any method may be used, for example, a method in which electrodeson which ink or paste prepared by dispersing carbon black carrying acatalyst such as platinum in a solution of ion exchange resin or asolution containing ion exchange resin and a binding agent is sprayed orapplied and then dried are laminated together and then hot pressed sothat an electrolyte membrane comes into contact with a catalyst layer; amethod in which electrolyte membranes on which ink or paste prepared bydispersing carbon black carrying a catalyst such as platinum in asolution of ion exchange resin or a solution containing ion exchangeresin and a binding agent is sprayed or applied and then dried arelaminated together and then hot pressed so that an electrode comes intocontact with a catalyst layer; a method in which releasing materials onwhich ink or paste prepared by dispersing carbon black carrying acatalyst such as platinum in a solution of ion exchange resin or asolution containing ion exchange resin and a binding agent is sprayed orapplied and then dried are laminated together and then hot pressed sothat an electrolyte membrane comes into contact with a catalyst layer,thereby the catalyst layer is transferred onto the electrolyte membrane,and then the resulting membranes are laminated and hot pressed so thatan electrode comes into contact with a catalyst layer; a method in whichan electrolyte membrane and an electrode are bonded using, as a binder,ink or paste prepared by dispersing carbon black carrying a catalystsuch as platinum in a solution of ion exchange resin or a solutioncontaining ion exchange resin and a binding agent; and a method in whichmaterials prepared by dispersing carbon black carrying a catalyst suchas platinum in a solution containing a binding agent such asfluororesin, applying the resultant to an electrode, drying and thenbaking are laminated and then hot pressed so that the electrode comesinto contact with a catalyst layer. During the hot pressing, membranesand/or electrodes may contain water, an organic solvent such as alcohol,an aqueous acid solution, and the like.

Use of the composite ion exchange membrane of the present inventionmakes it easy to produce an electrolyte membrane-electrode assemblybecause the membrane causes less swelling or less deformation during theproduction.

Many composite ion exchange membranes having a support membrane as asurface layer of an ion exchange membrane exhibit an adhesion toelectrodes inferior to that of ion exchange membranes composed only ofion exchange resin. However, the composite ion exchange membrane of thepresent invention can be bonded to an electrode easily like the ionexchange membranes composed only of ion exchange resin because it has onboth sides surface layers made of ion exchange resin containing nosupport membrane.

The electrolyte membrane-electrode assembly of the present invention hasa high ionic conductivity, but is superior in mechanical strength.Making the most of these characteristics, it is possible to use itparticularly as a solid polymer electrolyte membrane-electrode assemblyfor solid polymer fuel cells.

EMBODIMENTS

Examples of the present invention will be described below, which,however, in no way limit the scope of the present invention.

The evaluation methods and the analysis methods used in implementing thepresent invention are described below.

<Structural Observation by Transmission Electron Microscope>

The observation of a cross-sectional structure of a membrane by atransmission electron microscope (TEM) was carried out in the methoddescribed below. First, a sample slice for observation was prepared inthe manner provided below. That is, water inside a support membranesample after washing with water was displaced by ethanol, which was thenfurther displaced fully by epoxy monomer. The sample was held as it wasin epoxy monomer at 45° C. for six hours and was additionally heattreated at 60° C. for 20 hours to allow the epoxy to cure (epoxyembedding).

The sample thus epoxy embedded was sliced with a microtome equipped witha diamond knife into an ultrathin section having a thickness at whichthe interference colors exhibit from silver color to gold color. Theepoxy was removed by treating the section with a KOH-saturated ethanolsolution for 15 minutes (epoxy removal). Carbon was deposited onto thesample which was further washed with ethanol and then with water andsubsequently was stained with RuO₄. Then, the sample was observed by aTEM (JEM-2010) manufactured by JEOL at an acceleration voltage of 200kV.

<Structural Observation by Atomic Force Microscope>

The structural observation by an atomic force microscope (AFM) wascarried out in the method described below. Using an AFM (SPA300[observation mode: DFM mode, cantilever: SI-DF3; scanner: FS-100A]manufactured by Seiko Instruments Inc., structural observation of asurface of an undried support membrane held on a sample stage in waterwas carried out.

<Structural Observation by Scanning Electron Microscope>

The structural observation by a scanning electron microscope (SEM) wascarried out in the method described below. First, water in a supportmembrane sample washed with water was displaced by ethanol, which wasthen further displaced fully by isoamyl acetate. The resultant wassubjected to CO₂ supercritical point drying using a supercritical pointdrying apparatus (HCP-1) manufactured by Hitachi, Ltd. The supportmembrane thus supercritical point dried was applied with a platinumcoating with a thickness of 150 angstroms and then was observed at anacceleration voltage of 10 kV at a sample inclining angle of 30 degreesusing an SEM (S-800) manufactured by Hitachi, Ltd.

<Intrinsic Viscosity>

The viscosity of a polymer solution adjusted to have a concentration of0.5 g/L using methanesulfonic acid as solvent was measured with anUbbelohde's viscometer in a thermostat at 25° C. and then the intrinsicviscosity was calculated.

<Thickness of Support Membrane>

The thickness of an undried support membrane was measured by thefollowing method. Using a micrometer designed such that the measuringload can be changed, the thickness of a support membrane in water ateach load was measured. The value of an intercept obtained by plottingthickness measured versus load and extrapolating a linear portion to aload of zero was defined as thickness. The mean value of the thicknessesobtained by the measurement repeated at n=5 for one sample was used asthe thickness of the support membrane.

<Surface Open Area Ratio of Support Membrane>

The surface open area ratio of a support membrane was determined by thefollowing method. In a scanning electron microphotograph with amultiplication of 10,000 of the surface of the support membrane taken inthe way described above, a visual field corresponding to a square withsides having a length of 5 μm was chosen and was colored into white forportions corresponding to the outermost surface of the membrane and toblack for the other portions. Thereafter, the image was captured into acomputer through an image scanner. Using image analysis software ScionImage available from Scion Corp., U.S.A., the proportion accounted forby the black portions in the image was measured. This operation wasrepeated three times for one sample and the average was used as thesurface open area ratio.

<Porosity of Support Membrane>

The porosity of a support membrane was determined by the followingmethod. The volume Vw [mL] of the water filling the voids in themembrane can be obtained by dividing the weight of water calculated fromthe difference between the weight of a support membrane in awater-containing condition and an absolutely dried support membrane bythe density of water. The porosity of the support membrane wasdetermined from Vw and the volume of the membrane in a water-containingcondition Vm [mL] by a calculation shown below.Porosity of support membrane [%]=Vw/Vm×100<Thickness of Composite Ion Exchange Membrane and Thickness of LayersConstituting the Membrane>

The thickness of the composite layer constituting the composite ionexchange membrane and the thickness of the ion exchange resin layersformed on both sides of the composite layer so as to sandwich thecomposite layer were determined by photographing, by an opticalmicroscope, a section of the composite ion exchange membrane fullycooled in liquid nitrogen cut with a brand-new razor blade fully cooledin liquid nitrogen and then comparing it with a scale with a knownlength photographed at the same multiplication.

Regarding the thickness of the composite layer constituting thecomposite ion exchange membrane and the thickness of the surface layersformed of ion exchange resin free of support membranes, the surfacelayers being formed on both surfaces of the composite layer so as tosandwich the composite layer therebetween, a sample block was preparedby embedding a composite membrane sample cut into 300 μm in width and 5mm in length with resin having a composition of Luveak-812 (availablefrom Nakalai tesque)ALuveak-NMA (available from nakalai tesque)/DMP30(available from TAAB)=100/89/4 and then cured it at 60° C. for 12 hours.A tip of the block was cut with a diamond knife (SK2045 manufactured bySumitomo Electric Industries, Ltd.) using an ultramicrotome(2088Ultrotome manufactured by LKB) such that a smooth section wasexposed. The thicknesses were determined by photographing the section ofthe composite membrane thus exposed by an optical microscope and thencomparing it with a scale with a known length photographed at the samemultiplication. For example, in the case where the support has a largeporosity, there are some cases where no clear interface is formedbetween at least one surface layer and a composite layer arranged insidethe surface layer and the structure near the interface changescontinuously. In such cases, a portion closest to the outer surface ofthe composite ion exchange membrane, among the portions where acontinuous structural change can be confirmed by an optical microscopewas defined as the outermost surface of the composite layer and thedistance from it to the outer surface of the composite ion exchangemembrane was defined as the thickness of the surface layer. <IonExchange Resin (ICP) Content of Composite Ion Exchange Membrane>The ionexchange resin content of a composite ion exchange membrane wasdetermined by the following method. The weight Dc [g/m²] of a compositeion exchange membrane was measured. From the weight and the weight Ds[g/m²] of a dry support membrane measured by drying a support membranethe same as that used in the preparation of the composite ion exchangemembrane without being combined with ion exchange resin, the ionexchange resin content was determined by the calculation shown below.Ion exchange resin content [wt %]=(Dc−Ds)/Dc×100<Strength/Tensile Modulus>

The strength characteristic of an ion exchange membrane was measuredunder an atmosphere at a temperature of 25° C. and a relative humidityof 50% using a Tensilon manufactured by Orientech Co. The sample wasformed in a 10-mm wide strip-form. The characteristic was calculatedfrom a stress-strain curve measured at a span length of 40 mm and atensile speed of 20 mm/sec.

<Creep Elongation>

The creep elongation of an ion exchange membrane was determined by thefollowing method. By using a 10-mm wide strip-form membrane sample andplacing it under an atmosphere at a temperature of 80° C. and a relativehumidity of 95% at a span length of 50 mm under a load such that a loadof 8.1 MPa was applied to an initial cross-sectional area of themembrane sample, the creep elongation was determined from an amount ofdisplacement after 40 hours. The load is the amount of stress applied tothe membrane sample divided by the initial cross-sectional area of thecross section perpendicularly intersecting the loading direction of themembrane sample.

<High-Temperature Creep Elongation>

The high-temperature creep elongation of an ion exchange membrane wasdetermined by the following method. That is determined according to thecalculation formula shown below using a distance between chucks Lmeasured by setting a 5-mm wide strip-form membrane sample in a dryatmosphere at 130° C. at an initial span length L₀=25 mm under a loadsuch that a load of 1 MPa was applied to an initial cross-sectional areaof the membrane sample and then loading the sample for two hours.High-temperature creep elongation [%]={(L−L ₀)/L ₀}×100

The dry atmosphere is an air or nitrogen atmosphere dried so as to havea dew point of −30° C. or lower.

<Creep Elongation in High Temperature Water>

The creep elongation of an ion exchange membrane in high temperaturewater was determined by the following method. That is determinedaccording to the calculation formula shown below using a distancebetween chucks L measured by placing a 5-mm wide strip-form membranesample in water filled in a pressure resistant apparatus whose insidecan be observed while applying thereto a constant load which wasadjusted to be a load of 1 MPa to an initial cross section of themembrane sample at an initial span length L₀=25 mm.Creep elongation in high temperature water [%]={(L−L ₀)/L ₀}×100<Dimensional Change of Ion Exchange Resin>

The dimensional change of an ion exchange membrane between before andafter water absorption was measured by the following method. A squaresample with sides having a length of A cm was cut from an ion exchangemembrane vacuum dried at 110° C. for six hours. Assuming that after thesample was immersed in purified water at 80° C. for 24 hours to containwater, a longitudinal side and a transverse side be B cm and C cm inlength, respectively, the values calculated according to the followingformulas were defined as dimensional changes in the longitudinaldirection and the transverse direction. The longitudinal direction andthe transverse direction used herein are names for convenience regardingthe direction of the sample for dimensional change measurement and donot indicate specific directions of the membrane. However, when there isan apparent directivity in the production of the ion exchange membrane,it is convenient that the machine direction in the production be definedas the longitudinal direction.Longitudinal dimensional change [%]=((B−A)/A)×100   (Formula 2)Transverse dimensional change [%]=((C−A)/A)×100   (Formula 3)

The fact that a calculated result of a formula above is a positivenumber indicates that the length of the side has increased. On the otherhand, the fact that a calculated result is a negative number indicatesthat the length of the side has decreased.

<Gas Permeability>

The gas permeability of an ion exchange membrane was measured by thefollowing method. An ion exchange membrane was placed on a mesh-formsupport of stainless steel and was fixed to a holder. Then, helium gassaturated with water vapor was ventilated to one side of the ionexchange membrane so that the gauge pressure became 0.09 MPa and theamount of helium gas which passed through to the other side of the ionexchange membrane was measured and calculated using a soap film flowmeter.

<Ionic Conductivity>

The ionic conductivity σ was determined in the manner described below.Platinum wires (diameter: 0.2 mm) were pressed against the surfaces of a10-mm wide striped membrane sample on a self-made probe (made ofpolytetrafluoroethylene) for measurement. The sample was measured forthe alternating current impedance between the platinum wires at 10 kHzusing a 1250 Frequency Response Analyser manufactured by Solartron whilebeing held in a thermohygrostat at 80° C. and 95% RH The measurement wasconducted while the distance between the electrodes was varied from 10mm to 40 mm at intervals of 10 mm. From a slope Dr [Ω/cm] of a straightline obtained by plotting distances between the electrodes versusresistance measurements, an ionic conductivity was calculated accordingto the formula shown below while the contact resistance between themembrane and the platinum wires was cancelled.σ[S/cm]=1/(Membrane Width×Membrane Thickness [cm]×Dr)<Evaluation of Adhesion of Electrode Layer onto Ion Exchange Membrane>

The evaluation of the adhesion of an electrode layer onto an ionexchange membrane was conducted in the method described below. First, anelectrode paste was prepared by dispersing in 0.6 g of 10 wt % platinumcatalyst carried on carbon (EC-10-PRC) available from ElectroChem, Inc.,U.S.A. in 5 g of a 5 wt % Nafion solution available from Aldrich,U.S.A.. The catalyst paste was applied to sheets ofpolytetrafluoroethylene (PTFE) with an applicator and was dried in anoven at 100° C. for 10 minutes. Thus, electrode coating films 20 μm inthickness were formed on the PTFE sheets. The electrode coating filmseach formed on a PTEF sheet were allowed to face each other on bothsides of an ion exchange membrane and were subjected to hot press at130° C. under a pressure of 5 MPa for three minutes. Thus, electrodelayers were bonded to the surfaces of the ion exchange membrane.Subsequently, the PTFE sheet was removed. When delamination of electrodelayers was found in no one of three samples prepared in a manner thesame as that mentioned above, it is judged as being good in electrodeadhesion. If part or the whole of an electrode layer delaminated fromthe surface of an ion exchange membrane in any one of the three samples,it is judged as being poor in electrode adhesion.

<Electric Power Generation Characteristics>

The electric power generation characteristics of the electrolytemembrane-electrode assemblies produced by the methods described in thefollowing Examples and Comparative Examples were determined by themethod shown below. When an electrolyte membrane-electrode assembly wasincorporated in a fuel cell for evaluation FC25-02SP manufactured byElectroChem, Inc. and then an evaluation of current-voltagecharacteristic was conducted at a cell temperature of 80° C., a gashumidifying temperature of 80° C. at gas flow rates of 300 mL/min forhydrogen as fuel gas and 1000 mL/min for air as oxidizing gas underambient pressure, the current density expressed in the unit of [A/cm²]at a cell voltage of 0.2 V was used as the electric power generationcharacteristic of the electrolyte membrane-electrode assembly.

EXAMPLE 1

An isotropic solution with a poly(p-phenylene-cis-benzobisoxazole)concentration of 1% by weight was prepared. by diluting a dopecomprising polyphosphoric acid containing 14% by weight ofpoly(p-phenylene-cis-benzobisoxazole) polymer having IV=24 dL/g byaddition of methane sulfonic acid. This solution was formed into a filmon a glass plate heated to 90° C. at a film formation rate of 5 mm/secusing an applicator with a clearance of 300 μm. The thus obtained dopefilm was solidified on the glass plate used in the film formation in athermohygrostat at 25° C. and 80% RH for one hour and then was washedwith water until the washings exhibited pH 7±0.5, yielding a supportmembrane. The structural observation of the resulting support membraneconfirmed that it was a porous membrane with continuous pores havingopenings in both surfaces of the membrane. The support membrane wasfixed in a stainless steel frame in water and the water contained in thesupport membrane was displaced by a mixed solvent having a solventcomposition of water:ethanol:1-propanol=26:26:48 (weight ratio), whichis almost the same as that of a 20% Nafion (commercial name) solution(product number: SE-20192) manufactured by Du Pont, which is an ionexchange resin solution. The resulting support membrane was immersed ina 20% Nafion (commercial name) solution at room temperature for 15 hoursand then was removed from the solution. The solvent in the Nafion(commercial name) solution which permeated into the membrane or attachedto the surface of the membrane was volatilized to dry by air-drying. Thedried membrane was preheated for one hour in an oven at 60° C. forremoval of the remaining solvent and then was subjected to heattreatment for one hour at 150° C. under nitrogen atmosphere. Thus, acomposite ion exchange membrane of Example 1 was prepared.

EXAMPLE 2

A composite ion exchange membrane of Example 2 was prepared in the sameway as that described in Example 1 except that the rinsed supportmembrane was fixed in a stainless steel frame and then was immersed in a10% Nafion (commercial name) solution (product number: SE-10192)manufactured by Du Pont without displacing the water in the supportmembrane.

EXAMPLE 3

A composite ion exchange membrane of Example 3 was prepared in the sameway as that described in Example 1 except that the heat treatmenttemperature was changed to 130° C.

EXAMPLE 4

A composite ion exchange membrane of Example 4 was prepared in the sameway as that described in Example 3 except that the rinsed supportmembrane was fixed in a stainless steel frame and then was immersed in a10% Nafion (commercial name) solution (product number: SE-10192)manufactured by Du Pont without displacing the solution in the membrane.

EXAMPLE 5

A composite ion exchange membrane of Example 5 was prepared in the sameway as that described in Example 4 except that solidification wasconducted in a thermohygrostat at 25° C. and 80% RH for three minutes,followed by an additional solidification in purified water at 25° C. for15 minutes.

EXAMPLE 6

A composite ion exchange membrane of Example 6 was prepared in the sameway as that described in Example 4 except that the temperature of theglass plate and the film formation rate in the film formation werechanged to 70° C. and 50 mmjsec, respectively.

EXAMPLE 7

A composite ion exchange membrane of Example 7 was prepared in the sameway as that described in Example 1 except that a solution with apoly(p-phenylene-cis-benzobisoxazole) concentration of 1.5% by weightwas prepared by diluting a dope comprising polyphosphoric acidcontaining 14% by weight of poly(p-phenylene-cis-benzobisoxazole)polymer having IV=24 dL/g by addition of methane sulfonic acid.

EXAMPLE 8

A composite ion exchange membrane of Example 8 was prepared in the sameway as that described in Example 1 except that the film formation rateof the poly(p-phenylene-cis-benzobisoxazole) solution was changed to 10mm/sec. The composite ion exchange membrane prepared in this way had athickness of 50 μm. The thickness of the composite layer was 20 Am,which accounted for 40% of the thickness of the membrane.

EXAMPLE 9

A composite ion exchange membrane of Example 9 was prepared in the sameway as that described in Example 8 except that the rinsed supportmembrane was fixed in a stainless steel frame and then was immersed in a10% Nafion (commercial name) solution (product number: SE-10192)manufactured by Du Pont without displacing the water in the supportmembrane. The composite ion exchange membrane prepared in this way had athickness of 21 μm. The thickness of the composite layer was 11 μm,which accounted for 52% of the thickness of the membrane.

EXAMPLE 10

A composite ion exchange membrane of Example 10 was prepared in the sameway as that described in Example 8 except that the clearance of theapplicator in the film formation of thepoly(p-phenylene-cis-benzobisoxazole) solution was changed to 550 μm.The composite ion exchange membrane prepared in this way had a thicknessof 68 μm. The thickness of the composite layer was 62 μm, whichaccounted for 91% of the thickness of the membrane.

EXAMPLE 11

A composite ion exchange membrane of Example 11 was prepared in the sameway as that described in Example 8 except that the heat treatmenttemperature was changed to 130° C.

EXAMPLE 12

A composite ion exchange membrane of Example 12 was prepared in the sameway as that described in Example 10 except that the rinsed supportmembrane was fixed in a stainless steel frame and then was immersed in a10% Nafion (commercial name) solution (product number: SE-10192)manufactured by Du Pont without displacing the solution in the membrane.

EXAMPLE 13

A composite ion exchange membrane of Example 13 was prepared in the sameway as that described in Example 11 except that solidification wasconducted in a thermohygrostat at 25° C. and 80% RH for three minutes,followed by an additional solidification in purified water at 25° C. for15 minutes.

EXAMPLE 14

A composite ion exchange membrane of Example 14 was prepared in the sameway as that described in Example 11 except that the temperature of theglass plate and the film formation rate in the film formation werechanged to 70° C. and 50 mm/sec, respectively.

EXAMPLE 15

A composite ion exchange membrane of Example 15 was prepared in the sameway as that described in Example 8 except that a solution with apoly(p-phenylene-cis-benzobisoxazole) concentration of 1.5% by weightwas prepared by diluting a dope comprising polyphosphoric acidcontaining 14% by weight of poly(p-phenylene-cis-benzobisoxazole)polymer having IV=24 dL/g by addition of methane sulfonic acid.

EXAMPLE 16

A composite ion exchange membrane of Example 16 was prepared in the sameway as that described in Example 1 except that the isotropic solution ofpoly(p-phenylene-cis-benzobisoxazole) was formed into a film on a glassplate heated to 70° C.

EXAMPLE 17

A composite ion exchange membrane of Example 17 was prepared in the sameway as that described in Example 16 except that the rinsed supportmembrane was fixed in a stainless steel frame in water and then wasimmersed in a 10% Nafion (commercial name) solution (product number:SE-10192) manufactured by Du Pont without displacing the water in thesupport membrane.

EXAMPLE 18

A composite ion exchange membrane of Example 18 was prepared in the sameway as that described in Example 16 except that the heat treatmenttemperature was changed to 130° C.

EXAMPLE 19

A composite ion exchange membrane of Example 19 was prepared in the sameway as that described in Example 18 except that the rinsed supportmembrane was fixed in a stainless steel frame in water and then wasimmersed in a 10% Nafion (commercial name) solution (product number:SE-10192) manufactured by Du Pont without displacing the water in thesupport membrane.

EXAMPLE 20

A composite ion exchange membrane of Example 20 was prepared in the sameway as that described in Example 16 except that an isotropic solutionwith a poly(p-phenylene-cis-benzobisoxazole) concentration of 1.5% byweight was prepared by diluting a dope comprising polyphosphoric acidcontaining 14% by weight of poly(p-phenylene-cis-benzobisoxazole)polymer having IV=24 dL/g by addition of methane sulfonic acid and theresulting solution was used.

EXAMPLE 21

A composite ion exchange membrane of Example 21 was prepared in the sameway as that described in Example 1 except thatpoly(p-phenylene-cis-benzobisoxazole) polymer having IV=23 dL/g was usedin place of the poly(p-phenylene-cis-benzobisoxazole) polymer havingIV=24 dL/g.

EXAMPLE 22

A composite ion exchange membrane of Example 22 was prepared in the sameway as that described in Example 21 except that an isotropic solutionwith a poly(p-phenylene-cis-benzobisoxazole) concentration of 1.5% byweight was prepared by diluting a dope comprising polyphosphoric acidcontaining 14% by weight of poly(p-phenylene-cis-benzobisoxazole)polymer having IV=23 dL/g by addition of methane sulfonic acid and theresulting solution was used.

EXAMPLE 23

A composite ion exchange membrane of Example 23 was prepared in the sameway as that described in Example 1 except that the time for which thesolvent-replaced support membrane was immersed in a 20% Nafion(commercial name) solution was changed to one hour.

EXAMPLE 24

A composite ion exchange membrane of Example 24 was prepared in the sameway as that described in Example 23 except that the rinsed supportmembrane was fixed in a stainless steel frame in water and then wasimmersed in a 10% Nafion (commercial name) solution (product number:SE-10192) manufactured by Du Pont without displacing the water in thesupport membrane.

EXAMPLE 25

An isotropic solution with a poly(p-phenylene-cis-benzobisoxazole)concentration of 1% by weight was prepared by diluting a dope comprisingpolyphosphoric acid containing 14% by weight ofpoly(p-phenylene-cis-benzobisoxazole) polymer having IV=24 dL/g byaddition of methane sulfonic acid. This solution was formed into a filmon a glass plate heated to 70° C. at a film formation rate of 5mm/secusing an applicator with a clearance of 300 μm. The dope filmformed on the glass plate was placed as it was in a thermohygrostat at25° C. and 80% RH to be solidified for 10 minutes and was furthersolidified in purified water at 25° C. for additional 15 minutes. Theresulting film was washed with water until the washings exhibited pH7±0.5, yielding a support membrane. The surface morphology observationby an atomic force microscope and section morphology observation by atransmission electron microscope of the resulting support membraneconfirmed that it was a porous membrane with continuous pores havingopenings in both surfaces of the membrane. The support membrane wasfixed in a stainless steel frame in water and the water contained in thesupport membrane was displaced by a mixed solvent having a solventcomposition of water:ethanol:1-propanol=26:26:48 (weight ratio), whichis almost the same as that of a 20% Nafion (commercial name) solution(product number: SE-20192) manufactured by Du Pont, which is an ionexchange resin solution. The resulting support membrane was immersed ina 20% Nafion (commercial name) solution at 25° C. for 15 hours and thenwas removed from the solution. The solvent in the Nafion (commercialname) solution which permeated into the membrane or attached to thesurface of the membrane was volatilized to dry by air-drying. The driedmembrane was preheated for one hour in an oven at 60° C. for removal ofthe remaining solvent and then was subjected to heat treatment for onehour at 150° C. under nitrogen atmosphere. Thus, an ion exchangemembrane of Example 25 was prepared.

EXAMPLE 26

An isotropic solution with a poly(p-phenylene-cis-benzobisoxazole)concentration of 1% by weight was prepared by diluting a dope comprisingpolyphosphoric acid containing 14% by weight ofpoly(p-phenylene-cis-benzobisoxazole) polymer having IV=24 dL/g byaddition of methane sulfonic acid. This solution was formed into a filmon a glass, plate heated to 90° C. at a film formation rate of 20 mm/secusing an applicator with a clearance of 300 μm. The dope film formed onthe glass plate was placed as it was in a thermohygrostat at 25° C. and80% RH and was solidified for one hour. The resulting film was washedwith water until the washings exhibited pH 7±0.5, yielding a supportmembrane. The support membrane was fixed in a stainless steel frame inwater and the water contained in the support membrane was displaced by amixed solvent having a solvent composition ofwater:ethanol:1-propanol=26:26:48 (weight ratio), which is almost thesame as that of a 20% Nafion (commercial name) solution (product number:SE-20192) manufactured by Du Pont, which is an ion exchange resinsolution. The resulting support membrane was immersed in a 20% Nafion(commercial name) solution at 25° C. for 15 hours and then was removedfrom the solution. The solvent in the Nafion (commercial name) solutionwhich permeated into the membrane or attached to the surface of themembrane was volatilized to dry by air-drying. The dried membrane waspreheated for one hour in an oven at 60° C. for removal of the remainingsolvent and then was subjected to heat treatment for one hour at 150° C.under nitrogen atmosphere. Thus, a composite ion exchange membrane wasprepared. To a 20% Nafion (commercial name) solution (product number:SE-20192) manufactured by Du Pont, platinum-carrying carbon (carbon:ValcanXC-72 manufactured by Cabot Corp.; platinum carried: 40% byweight) was added so that the platinum:Nafion weight ratio became 2.7:1, and was stirred to yield a catalyst paste. The catalyst paste wasapplied to a carbon paper TGPH-060 manufactured by Toray Industries,Inc. so that platinum attached thereto in an amount of 1 mg/cm² and thendried. Thus, a gas diffusion layer with an electrode catalyst layer wasprepared. The aforementioned composite ion exchange membrane wassandwiched between two gas diffusion layers with an electrode catalystlayer in a manner that each electrode catalyst layer came into contactwith the membrane sample. Subsequently, they were applied with pressureand heat at 120° C. and 2 MPa for three minutes by hot pressing,yielding an electrolyte-electrode assembly of Example 26.

EXAMPLE 27

To a 20% Nafion (commercial name) solution (product number: SE-20192)manufactured by Du Pont, platinum-carrying carbon (carbon: ValcanXC-72manufactured by Cabot Corp.; platinum carried: 40% by weight) was addedso that the platinum:Nafion weight ratio became 2.7:1, and was stirredto yield a catalyst paste. The catalyst paste was applied to a compositeion exchange membrane the same as that provided in Example 25 so thatplatinum attached thereto in an amount of 1 mg/cm² and then dried. Thus,an ion exchange membrane with an electrode catalyst layer was prepared.The ion exchange membrane was sandwiched between two sheets of carbonpaper TGPH-060 manufactured by Toray Industries, Inc. Subsequently, theywere applied with pressure and heat at 120° C. and 2 MPa for threeminutes by hot pressing, yielding an electrolyte-electrode assembly ofExample 27.

EXAMPLE 28

A composite ion exchange membrane of Example 28 was prepared in the sameway as that described in Example 1 except that methane sulfonic acid wasadded to a dope comprising polyphosphoric acid containing 19% by weightof poly(2,6-diimidazo[4,5-b:4′,5′-e]pyridinylene-1,4(2,5-dihydroxy)phenylene) (henceforth abbreviated as PIPD) having anintrinsic viscosity of 18 dL/g to dilute it, yielding an isotropicsolution with a PIPD concentration of 1.5% by weight, which was used asa dope for use in preparation of a support membrane.

COMPARATIVE EXAMPLE 1

As Comparative Example 1, a commercially-available Nafion 112(commercial name) membrane manufactured by Du Pont was used. Thismembrane is a proton exchange membrane formed of perfluorocarbonsulfonic acid polymer the same as Nafion (commercial name) polymercontained in the 20% Nafion (commercial name) solution used in Example 1and the 10% Nafion (commercial name) solution used in Example 2. That iswidely used as a proton exchange membrane for solid polymer fuel cells.

COMPARATIVE EXAMPLE 2

A solution with a poly(p-phenylene-cis-benzobisoxazole) concentration of0.4% by weight was prepared by diluting a dope comprising polyphosphoricacid containing 14% by weight of poly(p-phenylene-cis-benzobisoxazole)polymer having IV=24 dL/g by addition of methane sulfonic acid. Thissolution was formed into a film on a glass plate heated to 90° C. at afilm formation rate of 5 mm/sec using an applicator with a clearance of300 μm. The thus obtained dope film was solidified on the glass plateused in the film formation in a thermohygrostat at 25° C. and 80% RH forone hour. However, the resulting support membrane was difficult to behandled thereafter because of its low strength.

COMPARATIVE EXAMPLE 3

A solution with a poly(p-phenylene-cis-benzobisoxazole) concentration of2.2% by weight was prepared by diluting a dope comprising polyphosphoricacid containing 14% by weight of poly(p-phenylene-cis-benzobisoxazole)polymer having IV=24 dL/g by addition of methane sulfonic acid. Despitean attempt to form this solution into a film on a glass plate heated to90° C. at a film formation rate of 5 mm/sec using an applicator with aclearance of 300 μm, it was impossible to form a uniform film due to thehigh viscosity of the solution.

COMPARATIVE EXAMPLE 4

A solution with a poly(p-phenylene-cis-benzobisoxazole) concentration of1% by weight was prepared by diluting a dope comprising polyphosphoricacid containing 14% by weight of poly(p-phenylene-cis-benzobisoxazole)polymer having IV=13 dL/g by addition of methane sulfonic acid. Thissolution was formed into a film on a glass plate heated to 90° C. at afilm formation rate of 5 mm/sec using an applicator with a clearance of300 Mm. The thus obtained dope film was solidified on the glass plateused in the film formation in a thermohygrostat at 25° C. and 80% RH forone hour. However, the resulting support membrane was difficult to behandled thereafter because of its low strength.

COMPARATIVE EXAMPLE 5

A solution with a poly(p-phenylene-cis-benzobisoxazole) concentration of1% by weight was prepared by diluting a dope comprising polyphosphoricacid containing 14% by weight of poly(p-phenylene-cis-benzobisoxazole)polymer having IV=36 dL/g by addition of methane sulfonic acid. Despitean attempt to form this solution into a film on a glass plate heated to90° C. at a film formation rate of 5 mm/sec using an applicator with aclearance of 300 Mm, it was impossible to form a uniform film due to thehigh viscosity of the solution.

COMPARATIVE EXAMPLE 6

A composite ion exchange membrane of Comparative Example 6 was preparedin the same way as that described in Example 4 except that a 15-minutesolidification in ethylene glycol at 25° C. was conducted in place ofthe 1-hour solidification in a thermohygrostat at 25° C. and 80% RH. Themeasurement of the ionic conductivity of the composite ion exchangemembrane of Comparative Example 6 was attempted. However, part of theion exchange resin layers formed on both surfaced of the composite layerpeeled under a high-temperature high-humidity atmosphere of 80° C. and95% RH and therefore the ionic conductivity could not be determined.

COMPARATIVE EXAMPLE 7

A composite ion exchange membrane of Comparative Example 7 was preparedin the same way as that described in Example 1 except that when thesupport membrane immersed in the Nafion (commercial name) solution waspicked up from the solution, the Nafion (commercial name) solutionattaching to both surfaces of the support membrane was scraped off witha scraper so that the surfaces of the solution would lose their luster.

COMPARATIVE EXAMPLE 8

A composite ion exchange membrane of Comparative Example 8 was preparedin the same way as that described in Example 9 except that the clearanceof the applicator in the film formation of thepoly(p-phenylene-cis-benzobisoxazole) solution was changed to 600 μm.The composite ion exchange membrane prepared in this way had a thicknessof 72 μm. The thickness of the composite layer was 70 μm, whichaccounted for 97% of the thickness of the membrane.

COMPARATIVE EXAMPLE 9

A composite ion exchange membrane of Comparative Example 9 was preparedin the same way as that described in Example 8 except that the clearanceof the applicator in the film formation of thepoly(p-phenylene-cis-benzobisoxazole) solution was changed to 30 μm. Thecomposite ion exchange membrane prepared in this way had a thickness of46 μm. The thickness of the composite layer was 2 μm, which accountedfor 4% of the thickness of the membrane.

COMPARATIVE EXAMPLE 10

A solution with a poly(p-phenylene-cis-benzobisoxazole) concentration of0.4% by weight was prepared by diluting a dope comprising polyphosphoricacid containing 14% by weight of poly(p-phenylene-cis-benzobisoxazole)polymer having IV=24 dL/g by addition of methane sulfonic acid. Thissolution was formed into a film on a glass plate heated to 90° C. at afilm formation rate of 10 mm/sec using an applicator with a clearance of300 μm. The thus obtained dope film was solidified on the glass plateused in the film formation in a thermohygrostat at 25° C. and 80% RH forone hour. However, the resulting support membrane was difficult to behandled thereafter because of its low strength.

COMPARATIVE EXAMPLE 11

A solution with a poly(p-phenylene-cis-benzobisoxazole) concentration of2.2% by weight was prepared by diluting a dope comprising polyphosphoricacid containing 14% by weight of poly(p-phenylene-cis-benzobisoxazole)polymer having IV=24 dL/g by addition of methane sulfonic acid. Despitean attempt to form this solution into a film on a glass plate heated to90° C. at a film formation rate of 10 mm/sec using an applicator with aclearance of 300 μm, it was impossible to form a uniform film due to thehigh viscosity of the solution.

COMPARATIVE EXAMPLE 12

A solution with a poly(p-phenylene-cis-benzobisoxazole) concentration of1% by weight was prepared by diluting a dope comprising polyphosphoricacid containing 14% by weight of poly(p-phenylene-cis-benzobisoxazole)polymer having IV=13 dL/g by addition of methane sulfonic acid. Thissolution was formed into a film on a glass plate heated to 90° C. at afilm formation rate of 10 mm/sec using an applicator with a clearance of300 μm. The thus obtained dope film was solidified on the glass plateused in the film formation in a thermohygrostat at 25° C. and 80% RH forone hour. However, the resulting support membrane was difficult to behandled thereafter because of its low strength.

COMPARATIVE EXAMPLE 13

A solution with a poly(p-phenylene-cis-benzobisoxazole) concentration of1% by weight was prepared by diluting a dope comprising polyphosphoricacid containing 14% by weight of poly(p-phenylene-cis-benzobisoxazole)polymer having IV=36 dL/g by addition of methane sulfonic acid. Despitean attempt to form this solution into a film on a glass plate heated to90° C. at a film formation rate of 10 mm/sec using an applicator with aclearance of 300 μm, it was impossible to form a uniform film due to thehigh viscosity of the solution.

COMPARATIVE EXAMPLE 14

A composite ion exchange membrane of Comparative Example 14 was preparedin the same way as that described in Example 11 except that a 15-minutesolidification in ethylene glycol at 25° C. was conducted in place ofthe 1-hour solidification in a thermohygrostat at 25° C. and 80% RH. Themeasurement of the ionic conductivity of the composite ion exchangemembrane of Comparative Example 14 was attempted. However, part of theion exchange resin layers formed on both surfaced of the composite layerpeeled under a high-temperature high-humidity atmosphere of 80° C. and95% RH and therefore the ionic conductivity could not be determined.

COMPARATIVE EXAMPLE 15

A composite ion exchange membrane of Comparative Example 15 was preparedin the same way as that described in Example 8 except that when thesupport membrane immersed in the Nafion solution was picked up from thesolution, the Nafion solution attaching to both surfaces of the supportmembrane was scraped off with a scraper so that the surfaces of thesolution would lose their luster.

COMPARATIVE EXAMPLE 16

A composite ion exchange membrane of Comparative Example 16 was preparedin the same way as that described in Example 16 except that when thesupport membrane immersed in the Nafion solution was removed from thesolution, the Nafion solution attaching to both surfaces of the supportmembrane was scraped off with a scraper made of polytetrafluoroethyleneso that the surfaces of the support membrane would be exposed.

COMPARATIVE EXAMPLE 17

A composite ion exchange membrane of Comparative Example 17 was preparedin the same way as that described in Example 16 except that after thevolatilization of the solvent in the Nafion solution which permeatedinto the support membrane or attached to the surface of the supportmembrane to dry by air-drying but before the heat treatment, a step ofimmersing the support membrane in a 20% Nafion solution at 25° C. forone minute followed by its removal further followed by volatilizing thesolvent in the Nafion solution to dry by air-drying was addedduplicately.

COMPARATIVE EXAMPLE 18

Using an isotropic solution having apolyparaphenylene-cis-benzobisoxazole concentration of 1% by weight thesame as that used in Example 16, a film was formed from this solution ona glass plate heated to 90° C. at a film formation rate of 5 mm/secusing an applicator with a clearance of 200 μm. The dope film formed onthe glass plate was placed as it was in a thermohygrostat at 25° C. and80% RH and was solidified for one hour. The resulting film was washedwith water until the washings exhibited pH 7±0.5, yielding a supportmembrane. This support membrane was fixed to a stainless steel frame inwater and was immersed in a solution at 25° C. for 15 hours, thesolution being adjusted to have a Nafion (commercial name) concentrationof 5% by weight by diluting a 10% Nafion (commercial name) solution(product number: SE-10192) by addition of purified water. Then, the filmwas removed from the solution and the solvent of the Nafion solutionwhich permeated into the membrane or attached to the surface of themembrane was volatilized to dry by air-drying. Moreover, a processincluding immersing the film at 25° C. for one minute in a 20% Nafion(commercial name) solution (product number: SE-20192) manufactured byDuPont, removing it and volatilizing the solvent of the Nafion solutionto dry by air-drying was repeated twice. Then, the resultant waspreheated for one hour in an oven at 60° C. for removal of the remainingsolvent and then was subjected to heat treatment for one hour at 150° C.under nitrogen atmosphere. Thus, a composite ion exchange membrane ofComparative Example 18 was prepared.

COMPARATIVE EXAMPLE 19

A composite ion exchange membrane of Comparative Example 19 was preparedin the same way as that described in Example 19 except that a 15-minutesolidification in ethylene glycol at 25° C. was conducted in place ofthe 1-hour solidification in a thermohygrostat at 25° C. and 80% RH. Themeasurement of the ionic conductivity of the composite ion exchangemembrane of Comparative Example 19 was attempted under atmosphere of 80°C. and 95% RH. However, part of the surface layers peeled with theabsorption of moisture by the composite ion exchange membrane andtherefore the ionic conductivity could not be determined.

COMPARATIVE EXAMPLE 20

An isotropic solution with a poly(p-phenylene-cis-benzobisoxazole)concentration of 0.4% by weight was prepared by diluting a dopecomprising polyphosphoric acid containing 14% by weight ofpoly(p-phenylene-cis-benzobisoxazole) polymer having IV=24 dL/g byaddition of methane sulfonic acid. This solution was formed into a filmon a glass plate heated to 90° C. at a film formation rate of 5 mm/secusing an applicator with a clearance of 300 μm. The thus obtained dopefilm was solidified on the glass plate used in the film formation in athermohygrostat at 25° C. and 80% RH for one hour. However, theresulting support membrane was difficult to be handled thereafterbecause of its low strength.

COMPARATIVE EXAMPLE 21

A solution with a poly(p-phenylene-cis-benzobisoxazole) concentration of2.2% by weight was prepared by diluting a dope comprising polyphosphoricacid containing 14% by weight of poly(p-phenylene-cis-benzobisoxazole)polymer having IV=24 dL/g by addition of methane sulfonic acid. Despitean attempt to form this solution into a film on a glass plate heated to90° C. at a film formation rate of 5 mm/sec using an applicator with aclearance of 300 μm, it was impossible to form a uniform film due to thehigh viscosity of the solution.

COMPARATIVE EXAMPLE 22

A composite ion exchange membrane of Comparative Example 22 was preparedin the same way as that described in Example 21 except that a 15-minutesolidification in ethylene glycol at 25° C. was conducted in place ofthe 1-hour solidification in a thermohygrostat at 25° C. and 80% RH. Themeasurement of the ionic conductivity of the composite ion exchangemembrane of Comparative Example 22 was attempted under atmosphere of 80°C. and 95% RH. However, part of the surface layers peeled with theabsorption of moisture by the composite ion exchange membrane andtherefore the ionic conductivity could not be determined.

COMPARATIVE EXAMPLE 23

A composite ion exchange membrane of Comparative Example 23 was preparedin the same way as that described in Example 23 except that when thesupport membrane immersed in the Nafion solution was removed from thesolution, the Nafion solution attaching to both surfaces of the supportmembrane was scraped off with a scraper made of Teflon (commercial name)so that the surfaces of the support membrane would be exposed.

COMPARATIVE EXAMPLE 24

A composite ion exchange membrane of Comparative Example 24 was preparedin the same way as that described in Example 24 except that after thevolatilization of the solvent in the Nafion solution which permeatedinto the support membrane or attached to the surface of the supportmembrane to dry by air-drying but before the heat treatment, a step ofimmersing the support membrane in a 20% Nafion solution at 25° C for oneminute followed by its removal further followed by volatilizing thesolvent in the Nafion solution to dry by air-drying was addedduplicately.

COMPARATIVE EXAMPLE 25

A drawn porous PTFE sheet having a thickness of 20 μm and a porosity of89% was fixed in a stainless steel frame and was immersed in a 20%Nafion (commercial name) solution at 25° C. for 15 hours. It was thenremoved from the solution and the solvent in the Nafion (commercialname) solution which permeated into the membrane or attached to thesurface of the membrane was volatilized to dry by air-drying. Theimpregnation/air-drying was repeated five times to form a membrane inwhich the pores and the surfaces of the drawn porous PTFE membrane werefilled with Nafion (commercial name) resin. The dried membrane waspreheated for one hour in an oven at 60° C. for removal of the remainingsolvent and then was subjected to heat treatment for one hour at 150° C.under nitrogen atmosphere. Thus, an ion exchange membrane of ComparativeExample 25 was prepared.

COMPARATIVE EXAMPLE 26

An ion exchange membrane with electrode catalyst layers was formed inthe same way as that described in Example 26 using acommercially-available Nafion 112 (commercial name) membranemanufactured by Du Pont as an ion exchange membrane in place of thecomposite ion exchange layer. This ion exchange membrane with electrodecatalyst layers was swollen to deform into a wavy form. Theaforementioned ion exchange membrane with electrode catalyst layers wassandwiched between two sheets of carbon paper TGPH-060 manufactured byToray Industries, Inc. Subsequently, they were applied with pressure andheat at 120° C. and 2 MPa for three minutes by hot pressing, yielding anelectrolyte-electrode assembly of Comparative Example 25. In thisassembly, since the membrane deformed to wave, bonding was achievedimperfectly.

COMPARATIVE EXAMPLE 27

An electrolyte-electrode assembly of Comparative Example 26 was preparedin the same way as that described in Example 25 except that when thesupport membrane immersed in the Nafion solution was removed from thesolution, the Nafion solution attaching to both surfaces of the supportmembrane was scraped off with a scraper made of polytetrafluoroethyleneso that the surfaces of the support membrane would be exposed. When thesupport membranes used in Examples 1 to 26 before being combined withion exchange resin were critical point dried in the way described in theExamples. The observation of their surface structure by SEM confirmedthat the support membranes had open pores defined by a network structureof fibril-like fibers. In FIG. 1, one example of an open pore structurein a surface of a support membrane is shown using a schematic view.

Table 1 summarizes the characteristics and the effects of the compositeion exchange membranes or the electrolyte membrane-electrode assembliesobtained in Examples 1 to 25, Comparative Example 1 and ComparativeExamples 2 to 27.

As shown in Examples 1 to 25 and 28, it is seen that if theconcentration of the dope used in the production of support membranesfalls within the range of from 0.5 to 2.0% by weight, a support membranewhich is uniform and has a strength sufficient to be handled can beobtained. In addition, when a dope having a concentration within such arange is formed into a film and solidified, a porous support membranehaving a surface open area ratio of 40% or more can be obtained by amethod comprising solidifying the film for a short time of 3 minutesunder a steam-containing atmosphere with a relative humidity of 80% at25° C. and then completing the solidification with water as well as amethod comprising solidifying the film for 60 minutes under asteam-containing atmosphere with a relative humidity of 80% at 25° C.

As shown in Comparative Examples 2, 3, 10, 11, 20 and 21, it is seenthat if a support membrane is produced at a dope concentration out ofthe range of from 0.5 to 2.0% by weight, there arise problems that nosupport membrane having a sufficient strength can be obtained or uniformfilm formation can not be achieved. On the other hand, as shown inComparative Examples 14, 19 and 22, it is seen that when an ethyleneglycol bath is used as a solidification bath for solidifying a supportmembrane, a phenomena such as small surface open area ratio are observedand no favorable support membrane can be obtained.

As shown in Comparative Examples 7, 16, 23 and 27, it is seen that in acomposite ion exchange membrane which is designed such that no ionexchange resin layer is formed on a surface of a support layer byscraping off an ion exchange resin solution adhering to the surface whenthe ion exchange resin is combined with the support membrane, part ofthe support is exposed in a surface and therefore, when a catalyst layeris bonded, the adhesion of the catalyst layer is affected, resulting inproblems of delamination of the catalyst layer and exhibition of lowelectric power generation characteristics.

It is seen that the composite ion exchange membranes provided in theExamples of the present invention exhibit improved durabilities, such asgreater tensile elongations, smaller creep elongations and smallerdimensional changes when containing water, in comparison with the Nafion112 (commercial name) membrane of Comparative Example 1 which containsno support membrane. Moreover, it is seen, as shown in Examples 1 to 20,that the composite ion exchange membranes of the present inventionexhibit significantly small values of gas permeability in comparisonwith Comparative Example 1 and therefore they are ion exchange membranessuperior in fuel permeation inhibitability. Furthermore, it is seen thatdespite containing supports, the composite ion exchange membranesprovided in the examples of the present invention do not exhibitsignificant deterioration in ionic conductivity or generationperformance in comparison with Comparative Example 1 containing nosupport.

From the features shown above, the composite ion exchange membrane ofthe present invention possesses excellent characteristics as a polymerelectrolyte membrane for fuel cells.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a sectional structure of a composite ionexchange membrane.

FIG. 2 is a schematic view of a sectional structure of a composite ionexchange membrane on which catalyst layers are bonded.

FIG. 3 is a schematic view of an image obtained by critical point dryinga support membrane before being combined with ion exchange resin andthen observing a surface of the support membrane by a scanning electronmicroscope.

EXPLANATION OF REFERENCE NUMERALS

1 Surface layer A, 2 Composite layer 3 Surface layer B,

4 Catalyst layer, 5 Gas diffusion layer,

6 Fibril of support membrane, 7 Void

Industrial Applicability

It is possible to offer an electrolyte membrane-electrode assembly thatincludes a solid polymer electrolyte membrane which has high mechanicalstrength and which is superior in ionic conductivity, electric powergeneration characteristic and gas barrier property and that exhibitsgood adhesion between the electrolyte membrane and the electrodeassembly. TABLE 1 Total thickness of Composite Surface composite layerSurface Surface open area ion Thickness thickness/ ICP layer ICP layerICP Thickness ratio of Porosity of exchange of Total thickness thicknesscontent Ionic Breaking of support support support membrane compositemembrane A B % by conductivity strength μm % % μm layer thickness μm μmweight S/cm MPa Example 1 90 69 98 50 15 95 0.19 27 Example 2 90 69 9821 5 89 0.16 41 Example 3 90 69 98 50 15 95 0.22 26 Example 4 90 69 9821 5 89 0.19 39 Example 5 55 54 96 21 4 85 0.17 65 Example 6 90 68 98 204 84 0.17 43 Example 7 105 44 97 52 12 98 0.17 38 Example 8 90 69 97 5020 40 98 0.19 27 Example 9 90 69 97 21 11 52 89 0.16 41 Example 10 17769 97 68 62 91 86 0.17 53 Example 11 90 69 97 50 20 40 95 0.22 26Example 12 90 69 98 21 11 52 89 0.19 39 Example 13 55 54 96 21 13 62 850.17 65 Example 14 90 68 98 20 12 60 84 0.17 43 Example 15 105 44 97 5228 54 95 0.17 38 Example 16 69 50 19 17 14 95 0.19 28 Example 17 69 2111 5 5 89 0.16 41 Example 18 69 50 19 16 15 95 0.22 24 Example 19 69 2111 5 5 89 0.19 39 Example 20 44 52 28 12 12 95 0.17 38 Example 21 69 5019 17 14 95 0.19 28 Example 22 44 52 28 12 12 95 0.17 36 Example 23 6948 18 16 14 93 0.19 29 Example 24 69 18 10 4 4 87 0.16 44 Example 25 4848 0.19 Example 26 48 19 16 13 94 0.19 28 Example 27 48 19 16 13 94 0.1828 Example 28 113 51 54 30 13 11 93 0.18 32 Creep High- elongationtemperature in high Dimensional Dimensional Gas Tensile Creep creeptemperature change in change in permeability Electrode moduluselongation elongation water length width Cm² · cm/ adhesion GenerationMPa % % % % % cm² · s · MPa — performance Example 1 716 28 1.6 × 10⁻⁶Good Example 2 1429 25 1.6 × 10⁻⁶ Good Example 3 766 25 1.4 × 10⁻⁶ GoodExample 4 1406 22 1.4 × 10⁻⁶ Good Example 5 2545 21 1.2 × 10⁻⁶ GoodExample 6 2205 23 1.4 × 10⁻⁶ Good Example 7 1220 15 1.4 × 10⁻⁶ GoodExample 8 716 28 1.6 × 10⁻⁶ Good Example 9 1429 25 1.6 × 10⁻⁶ GoodExample 10 2520 20 1.2 × 10⁻⁶ Good Example 11 766 25 1.4 × 10⁻⁶ GoodExample 12 1406 22 1.4 × 10⁻⁶ Good Example 13 2545 21 1.2 × 10⁻⁶ GoodExample 14 2205 23 1.4 × 10⁻⁶ Good Example 15 1220 15 1.4 × 10⁻⁶ GoodExample 16 718 1.6 × 10⁻⁶ 0.7 Example 17 1429 1.6 × 10⁻⁶ 0.7 Example 18766 1.4 × 10⁻⁶ 0.7 Example 19 1406 1.4 × 10⁻⁶ 0.75 Example 20 1220 1.4 ×10⁻⁶ 0.7 Example 21 699 0.7 Example 22 1188 0.7 Example 23 722 0 0.5Example 24 1542 0.2 0 Example 25 2 12 Example 26 724 1.6 × 10⁻⁶ 0.7Example 27 724 1.6 × 10⁻⁶ 0.6 Example 28 922ICP: Ion exchange resin

TABLE 2 Total thickness of Composite Surface composite layer SurfaceSurface open area Porosity ion Thickness thickness/ ICP layer ICP layerICP Thickness ratio of of exchange of Total thickness thickness contentIonic Breaking of support support support membrane composite membrane AB % by conductivity strength μm % % μm layer thickness μm μm weight S/cmMPa Comparative No support 49 No support 100 0.2  22 Example 1Comparative 75 16 82 19 4 Undeterminable Example 6 Comparative 90 69 9815 0 62 0.12 Example 7 Comparative 171 69 98 72 70 97 85 0.18 55 Example8 Comparative 10 68 98 45 2 4 98 0.20 20 Example 9 Comparative 75 16 8219 11 58 Undeterminable Example 14 Comparative 90 69 98 15 15 100 620.12 Example 15 Comparative 69 25 25 0 0 46 0.04 144 Example 16Comparative 69 139 20 61 58 98 0.19 25 Example 17 Comparative 64 99 4 4847 98 0.19 25 Example 18 Comparative 16 19 11 4 4 72 UndeterminableExample 19 Comparative 16 19 11 4 4 72 Undeterminable Example 22Comparative 69 22 22 100 0 0 44 0.04 15 Example 23 Comparative 69 137 2015 60 57 95 0.19 22 Example 24 Comparative 42 0.16 Example 25Comparative 50 No support 100 0.2  22 Example 26 Comparative 24 24 0 043 0.04 146 Example 27 Creep High- elongation temperature in highDimensional Dimensional Tensile Creep creep temperature change in changein Gas permeability Electrode modulus elongation elongation water lengthwidth Cm² · cm/ adhesion Generation MPa % % % % % cm² · s · MPa —performance Comparative 315 150 200 284 −7 26 3.6 × 10⁻⁶ Good 0.8Example 1 Comparative Example 6 Comparative Delaminated Example 7Comparative 2662 19 Delaminated Example 8 Comparative 334 34 GoodExample 9 Comparative Example 14 Comparative Delaminated Example 15Comparative 2825 0.2 Example 16 Comparative 402 Surface ICP layerExample 17 delaminated. Undeterminable Comparative 488 Surface ICP layerExample 18 delaminated. Undeterminable Comparative Example 19Comparative Example 22 Comparative 2883 −0.1 −0.1 Delaminated Example 23Comparative 402 0.7 1 Example 24 Comparative 72 85 Example 25Comparative 317 3.6 × 10⁻⁶ Delaminated 0.5 Example 26 Comparative 30112.0 × 10⁻⁶ Delaminated 0.2 Example 27ICP: Ion exchange resin

1. A composite ion exchange membrane including a composite layercomprising a support membrane with continuous voids formed ofpolybenzazole polymer, the support layer being impregnated with ionexchange resin, and surface layers formed of ion exchange resin free ofsupport membranes, the surface layers being formed on both surfaces ofthe composite layer so as to sandwich the composite layer therebetween.2. The composite ion exchange membrane according to claim 1, wherein thethickness of the composite layer accounts for from 5% to 95% of thetotal thickness of the composite ion exchange membrane.
 3. The compositeion exchange membrane according to claim 1, wherein each of the surfacelayers has a thickness of from 1 μm to 50 μm and does not exceed halfthe total thickness of the composite ion exchange membrane.
 4. Acomposite ion exchange membrane including a composite layer comprising asupport membrane with continuous voids formed of polybenzazole polymer,the support layer being impregnated with ion exchange resin, and surfacelayers formed of ion exchange resin free of support membranes, thesurface layers being formed on both surfaces of the composite layer soas to sandwich the composite layer therebetween, wherein at least onesurface of the support membrane has an open porosity of 40% or more. 5.The composite ion exchange membrane according to claim 1, wherein thedimensional change, based on the length of each side of a dry compositeion exchange membrane cut into a square in an arbitrary direction in amembrane surface, of each corresponding side of the composite ionexchange membrane impregnated with water through its 24-hour immersionin pure water at 80° C. is within the range between a 5% decrease and a10% increase.
 6. The composite ion exchange membrane according to claim1, wherein the composite ion exchange membrane exhibits ahigh-temperature creep elongation of up to 50% when it is applied with aload of 1 MPa for two hours under a dry atmosphere at 1 30° C.
 7. Thecomposite ion exchange membrane according to claim 1, wherein thecomposite ion exchange membrane exhibits a creep elongation in hightemperature water of up to 60% when it is applied with a load of 1 MPafor two hours in water at 130° C.
 8. A method for producing the supportmembrane according to claim 1, the method comprising forming apolybenzazole polymer solution into a film form and then solidifying it,wherein the polybenzazole polymer solution is an isotropic solutioncontaining the polybenzazole polymer in an amount of from 0.5% by weightto 2% by weight.
 9. An electrolyte membrane-electrode assemblycomprising a composite ion exchange membrane including a composite layercomprising a support membrane with continuous voids formed ofpolybenzazole polymer, the support membrane being impregnated with ionexchange resin, and surface layers formed of ion exchange resin free ofsupport membranes, the surface layers being formed on both surfaces ofthe composite layer so as to sandwich the composite layer therebetween,wherein catalyst layers and gas diffusion layers are disposed on bothsurfaces of the composite ion exchange membrane.
 10. The electrolytemembrane-electrode assembly according to claim 9, wherein each of thesurface layers formed of ion exchange resin free of support membranes,the surface layers being formed on both surfaces, has a thickness offrom 1 μm to 50 μm and does not exceed half the total thickness of thecomposite ion exchange membrane.
 11. The electrolyte membrane-electrodeassembly according to claim 9, wherein at least one surface of thesupport membrane has an open porosity of 40% or more.
 12. A fuel cellusing the electrolyte membrane-electrode assembly according to claim 9.13. The composite ion exchange membrane according to claim 4, whereinthe dimensional change, based on the length of each side of a drycomposite ion exchange membrane cut into a square in an arbitrarydirection in a membrane surface, of each corresponding side of thecomposite ion exchange membrane impregnated with water through its24-hour immersion in pure water at 80° C. is within the range between a5% decrease and a 10% increase.
 14. The composite ion exchange membraneaccording to claim 4, wherein the composite ion exchange membraneexhibits a high-temperature creep elongation of up to 50% when it isapplied with a load of 1 MPa for two hours under a dry atmosphere at130° C.
 15. The composite ion exchange membrane according to claim 4,wherein the composite ion exchange membrane exhibits a creep elongationin high temperature water of up to 60% when it is applied with a load of1 MPa for two hours in water at 1 30° C.
 16. A method for producing thesupport membrane according to claim 4, the method comprising forming apolybenzazole polymer solution into a film form and then solidifying it,wherein the polybenzazole polymer solution is an isotropic solutioncontaining the polybenzazole polymer in an amount of from 0.5% by weightto 2% by weight.